Rotating a water wheel
When only you animate frame 1 to 60, it means that it will not start at frame 1 with full speed and it will not end at frame 60 with full speed, butthe motion will get faster in the beginning and slower in the end. Go to Graph Editor and look how the rotation-curve is getting steeper.  

To counter that you can start the animation earlier and end later. Or you manipulate the development of the roation in den Graph Editor, making it a constant movement.